This attack started with a well known pattern where protocols where you can freely mutate cyphertext in transit usually end of building a decryption oracle into them that an attacker can exploit.

Then they found a novel oracle in the image decryption system.

Then big change at Apple lately is that on top of having world class cypto and security people they are publishing a lot more of their design work for peer review. This will lead to much more secure systems when they replace the iMessage crypto.